On going studies on the mechanism of action of coumarin anticoagulants have implicated glycosylation of the proteins of the prothrombin complex as a likely site of action. Glycosyl transferase activity of rat liver golgi preparations were inhibited by anticoagulants in a dose related manner. The degree of transferase inhibition was dependent on factors involving (a) optimal manganese concentration, (b) total enzyme activity of the preparation, i.e. triton treated, lysolecithin activated, or nontreated enzyme. (c) Nature of the acceptor, endogenous or exogenous, (d) presence of EDTA. The concentration of courmarin anticoagulant 2-5x10-4M required for 50% inhibition of golgi glycosyl transferase activity is approximately the same concentration required in previous studies to show similar inhibition by anticoagulant treatment at other levels of cellular organization, e.g. glycoprotein synthesis in liver slices, ESR studies on Golgi-Mn interactions.